drm/i915/selftests: Wait until we start timeslicing after a submit

If we submit, we do not start timeslicing until we process the CS event
that marks the start of the context running on HW. So in the selftest,
be sure to wait until we have processed the pending events before
asserting that timeslicing has begun.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200403190209.21818-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/selftest_lrc.c

@@ -1244,7 +1244,12 @@ static int live_timeslice_queue(void *arg)
 		if (err)
 			goto err_rq;
 
+		/* Wait until we ack the release_queue and start timeslicing */
+		do {
+			cond_resched();
 			intel_engine_flush_submission(engine);
+		} while (READ_ONCE(engine->execlists.pending[0]));
+
 		if (!READ_ONCE(engine->execlists.timer.expires) &&
 		    !i915_request_completed(rq)) {
 			struct drm_printer p =